github auth without user entity should be an optional resolver

[jkleinlercher]

follow up for #646 (comment)

with the following commits in our backstage app:

https://github.com/suxess-it/sx-backstage/commit/e64d0f12f4d02b15f8a9adba2f226fea27e0f21d
https://github.com/suxess-it/sx-backstage/commit/f0099dd6f895b7782d7e94e5daa989704207b6e1
https://github.com/suxess-it/sx-backstage/commit/67a46975f1ec6b418ba8de043a1bffcb18a842be

every github account can login to backstage without the need that this user must be an entity in our backstage catalog. This helps for the kubriX playground to be easier to use. However, this is now implemented also in the backstage instance for uibklab because it is in the official build. For now, this is no problem, but in the future we want this "github without user entity" resolver to be optional. Then we can activate it only in the codespaces instance, but not everywhere.